The invention concerns a hydraulic steering arrangement with a steering handwheel which is connected with a hydraulic steering unit, and with a steering motor connected with a pump via a steering valve.
A steering arrangement of this kind is known from U.S. Pat. No. 5,234,070. In dependence of the direction determined by the steering handwheel, the steering valve releases a connection between the pump and the steering motor, which finally causes the angle movement of the steered wheels of a vehicle. For this purpose, an angle sensor, connected with a microprocessor, is arranged on the steering hand-wheel. This microprocessor controls electromagnetic drives, displacing the slide of the steering valve in one direction or the other. On failure of the electromagnetic drives, the slide of the steering valve can also be displaced by a hydraulic pressure, which is produced by a pump connected with the steering handwheel, when the steering handwheel is turned.
An embodiment of this kind meets certain security requirements, as a vehicle equipped in that manner can still be steered when the electrical components fail. However, the embodiment of the steering valve is relatively complicated.
It is the task of the invention to provide a simple embodiment of a steering arrangement though securing a safe operation.
In a hydraulic steering arrangement of the kind described in the introduction, this task is solved in that optionally the steering valve can be operated by a steering handwheel or by a hydraulic pressure generated by a control pressure generator.
Thus, the embodiment of the steering valve becomes relatively simple. Merely a connecting arrangement must be provided, via which the hydraulic pressure for the control of the steering valve can act upon the operating element, for example a slide. A synchronisation between an electromagnetic additional drive and a hydraulic drive is not required. Accordingly, it must be assumed that the switching from a xe2x80x9cservo operationxe2x80x9d, by which the hydraulic pressure comes from the control pressure generator, to the xe2x80x9cmanual operationxe2x80x9d, by which the hydraulic pressure is produced by the steering handwheel, is less felt by the driver.
In a preferred embodiment it is provided that the control pressure generator converts an electrical input signal to a hydraulic control pressure. Thus, all the control opportunities also obtained with a direct electromagnetic drive of the steering valve slide are maintained. For example, the control pressure generator can be controlled by a microprocessor or a remote control. A control pressure generator is manufactured by the applicant, for example under the name PVE.
Preferably, the steering handwheel has a sensor, which is connected with the control pressure generator. Thus, the steering handwheel cannot only be used for building up the hydraulic pressure required for the operation of the steering valve in the case of a failure. By means of the sensor, the signals to be used for the steering of the vehicle, can be obtained directly. Thus, a plurality of control algorithms can be realised. This is particularly advantageous, when the steering arrangement is to be used in driven machines, for example fork lift trucks. In this connection, the term xe2x80x9csteering handwheelxe2x80x9d does not only mean the part actually operated by the driver, but also all parts connected with it, for example the steering column.
Preferably, for each movement direction of its slide the steering valve is connected with an outlet of a shuttle valve, whose one inlet is connected with the steering unit and whose other inlet is connected with the control pressure generator. With a shuttle valve of this kind, the switching from servo operation to manual operation can be automated. The shuttle valve always passes on the higher pressure of its two inlets to the outlet. On a failure of the control pressure generator, for example caused by a failure in the electrical energy supply, it stops producing hydraulic pressure. Accordingly, the pressure produced by the steering unit is larger on a steering movement of the steering handwheel. This pressure is then passed on to the control inlets of the steering valve in order to operate the slide.
Preferably, the pressure produced by the steering unit is larger than the pressure produced by the control pressure generator. In this case, it will still be possible also to intervene in the steering of the vehicle, when the control pressure generator is working. An intervention of this kind may be required in an emergency situation. As the pressures produced by the steering unit are larger than the pressures produced by the control pressure generator, an operation of the steering unit will cause a switching of the belonging shuttle valve. As the shuttle valve makes a unique choice of the pressure to be passed on to its outlet, an operation of the steering handwheel will cause that the control pressure generator no longer has any influence on the steering behaviour of the vehicle. Preferably, the pump inlet of the steering valve is connected with the outlet of a shuttle valve, whose one inlet has a connection with the pump and whose other inlet has a connection with the steering unit. This adds a third step to the security of the steering unit. A steering operation is namely also provided, when also the pump (or the pressure source available in the vehicle) fails. Normally, the pump produces the pressure required for the operation of the pump. As long as the pump works, the shuttle valve passes on the pump pressure to the steering valve. The connection to the steering handwheel or the steering unit, respectively, is, however, interrupted by the shuttle valve, so that there is no risk of negative effects of the pump pressure on the steering unit. When, however, the pump pressure drops below the pressure produced by the steering unit, the pressure produced by the steering unit will be used to operate the steering motor. As such a pressure production only occurs, when the steering valve has been operated by the steering unit, a connection between the steering unit and the steering motor also exists via the shuttle valve and the steering valve. This connection is interrupted, when the steering handwheel is no longer turned, so that there is no risk of a counter-effect of the steering motor on the steering handwheel.
Preferably, the connection between the steering unit and the shuttle valve is interrupted in the neutral position of the steering valve. This is an additional security measure. A connection is only set free, when a pressure actually has to be passed on. In the neutral position of the steering valve, counter-effects on the steering handwheel are avoided.
Preferably, the connection between the steering unit and the shuttle valve is led through the steering valve. This makes it easier to ensure that the connections between the steering unit and the shuttle valve on the one hand and between the shuttle valve and the steering motor on the other hand are always interrupted or released simultaneously.
In a particularly simple embodiment, this is preferably obtained in that both the connection between the steering unit and the shuttle valve and the connection between the shuttle valve and the steering motor are controlled by the same valve slide. A movement of the valve slide, which is required to produce a connection between the pump and the steering motor, will automatically also produce the connection between the steering unit and the shuttle valve. When, however, the slide returns to its neutral position, not only the first connection, but also the second connection, is interrupted.
Preferably, a pilot pressure valve, controlled by a load sensing pressure, is arranged between the connection of steering unit and shuttle valve and a pressure sink. With this pilot pressure valve it can be obtained that the signal pressure generator can provide relatively small control pressures, also when higher pressures rule in the load sensing pressure line (LS line). This is normally the case, as there the pressure from the steering motor is used to control the belonging pump. By means of the pilot pressure valve, the corresponding excess pressure can be reduced by returning hydraulic fluid to the tank.